Although the primary intended use of this invention is on large trucks and tractor-trailer combinations, it should be appreciated that the invention is also suitable for use on all vehicles which have independent brakes for each wheel, such as vehicles which generally incorporate an air-brake system. These brakes must be properly adjusted in order properly to function. Various standards have been set by state governments and the Federal Department of Transportation (DOT) to insure that brakes are properly adjusted and are therefore operable to stop these heavy vehicles when the need arises.
One standard that has been set by the DOT is a requirement that the travel of an actuator rod, which extends from a brake air cylinder to an activation arm on the brake mechanism, have a travel distance of two inches or less. A brake is deemed to be out of adjustment if the activation arm travel is two or more inches. Travel distances between 1.75 inches and two inches are considered to be marginally safe, although a travel distance of less than 1.75″ is preferable.
Newer vehicles have self-adjusting brakes. It is estimated that, at this time, approximately fifty percent of the vehicles on the road have such self adjustors. Even with the self-adjusting brakes, the slack adjustors, as they are known in the trade, will sometimes fail to operate or will become inoperative as a result of the build up of petrochemical products, dirt or ice in the slack adjustment mechanism, which allows brakes to be out of adjustment. Older vehicles do not have any self-adjustment mechanism. Regardless of whether the vehicle is equipped with slack adjustors or not, it is a requirement that the driver physically inspect the brakes on the tractor and trailer(s) to insure that they are properly adjusted and are properly operating. As might be expected, this task is not always performed, particularly in inclement weather conditions. If, however, a brake is determined to be out of adjustment, it may be quickly adjusted by the truck operator, with a minimal expenditure of energy and with the use of a few hand tools.
Vehicle brakes are inspected at check points, such as the familiar weigh stations which are found along highways. As in the case of the driver, an inspector must generally crawl under the truck to inspect the travel of the actuator arms to insure that they are in compliance with federal and state regulations. Such inspection must be done for each brake on the vehicle, which generally requires that the driver remain in the truck and operate the brakes while the inspector visually checks each brake mechanism.
A number of monitoring devices have been provided for use on trucks. Some of these monitor air pressure to determine if the brakes are operating safely, others provide a warning only after the brake travel has exceeded legal limits, while still others provide a purely visual indication which still requires the operator to visual check every brake on the truck.
There have been a few attempts to build and market such systems in the industry, however, all known systems suffer from one or more serious defects. Some require extensive and expensive modifications to the vehicle and brake system, others are difficult or inconvenient to use, and still others are not readily adaptable to the wide variety of axle configurations found on modern trucks.
What is needed then is a automatic brake-travel monitoring system that is simple and reliable, easy for the operator to use, is inexpensive to install and maintain, and is readily adaptable to various axle configurations.
U.S. Pat. No. 5,285,190 for Automatic slack adjuster with operation and adjustment monitor, to Humphries, et al., granted Feb. 8, 1994, disclosed an automatic slack adjustor which incorporates a monitoring system alerting the vehicle operator that a brake somewhere on the vehicle is not operating properly, or is out of adjustment, but does not provide any mechanism for identifying the specific brake that is out of adjustment, nor does the system identify any quantitative information about a particular brake.
U.S. Pat. No. 4,937,554 for Electronic brake monitoring system and method, to Herman, granted Jun. 26, 1990, provides a monitoring system which incorporates a push rod oscillator-type sensor to provide an analog output which is related to the push rod extension distance, which output is used along with pressure in the brake system to determine whether or not a brake is operating properly.
U.S. Pat. No. 4,800,991 for Brake maintenances monitor to Miller, granted Jan. 31, 1989, discloses a system which utilizes a mechanical flag to provide an indication that the brake actuator arm has exceeded its safe distance. Additionally, an electronic warning device is provided which will momentarily provide a warning light on the dashboard of the vehicle.
U.S. Pat. No. 5,433,296 for Brake monitoring system, to Webberley, granted Jul. 18, 1995, discloses a system which provides a readout to an operator of a motor vehicle.